While a number of efforts are currently being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, deep-ultraviolet lithography is thought to hold particular promise as the next generation in microfabrication technology. Deep UV lithography enables micropatterning to a feature size of 0.3 or 0.4 μm. One technology that has attracted a good deal of attention recently utilizes as the deep UV light source a high-intensity KrF excimer laser, especially an ArF excimer laser featuring a shorter wavelength. There is a desire to have a resist material capable of micropatterning to a smaller feature size.
In this regard, the recently developed, acid-catalyzed, chemical amplification type positive resist materials (see JP-B 2-27660 and JP-A 63-27829) are expected to comply with the deep UV lithography because of their many advantages including high sensitivity, resolution and dry etching resistance.
On use of the chemical amplification type resist compositions, especially chemical amplification type, positive working resist compositions, a resist film is formed by dissolving a resin having acid labile groups as a binder and a compound capable of generating an acid upon exposure to radiation (to be referred to as photoacid generator) in a solvent, applying the resist solution onto a substrate (inclusive of a stepped substrate) by a variety of methods, and evaporating off the solvent optionally by heating. The resist film is then exposed to radiation, for example, deep UV through a mask of a predetermined pattern. This is optionally followed by post-exposure baking (PEB) for promoting acid-catalyzed reaction. The exposed resist film is developed with an aqueous alkaline developer for removing the exposed area of the resist film, obtaining a positive pattern profile. The pattern profile of resist is then transferred to the substrate by dry or wet etching. Since the stepped substrate and the aligner used in device fabrication have more or less errors, there is a desire to have a resist material capable of forming an accurate pattern even when the focal point is somewhat offset, i.e., having a wide depth of focus.
Several acid labile group-substituted resins are known suitable for use in chemically amplified positive resist compositions. Included are resins protected with t-butyl ester groups or t-butoxycarbonyl groups (JP-B 2-27660 referred to above), resins protected with 1-ethoxyethyl groups (JP-A 5-249682 and JP-A 6-308437), and resins protected with t-butoxycarbonyl groups and 1-alkoxyethyl groups (JP-A 8-123032). These chemically amplified resist compositions, however, have their own problems. A variety of difficulties arise on the practical application of these compositions. Such problems include, for example, environmental stability, focal latitude, particle, and storage stability.
The environmental stability is generally divided into two categories. One environmental stability is related to the deactivation of a photo-generated acid by an air-borne base above the resist film or a base beneath the resist film and on the substrate. This phenomenon is often seen when a photoacid generator capable of generating an acid having a high acid strength is used. It is expected that this problem is solved by introducing into the resin acid labile groups which are more easy to cleavage by acid or by lowering or weakening the acid strength of the photo-generated acid. The other environmental stability is that when the period from exposure to post-exposure baking (PEB) is prolonged, which is known as post-exposure delay (PED), the photo-generated acid diffuses in the resist film so that acid deactivation may occur when the acid labile groups are less susceptible to scission and acid decomposition may take place when the acid labile groups are susceptible to scission, often inviting a change of the pattern profile in either case. For example, this often invites a narrowing or slimming of the line width in the unexposed area in the case of chemical amplification type, positive working, resist compositions having acid labile groups mainly of acetal type.
Of the above-referred protective groups, the t-butoxycarbonyl groups have poor environmental stability on the surface of resist film or on the substrate (i.e., at the interface between the resist and the substrate). As a result, the pattern obtained can have an outward extending top (T-top profile) or is not sharply defined at all. Alternatively, pattern footing and tapering are sometimes possible. Also the resolving power is too low to provide a finer pattern.
The use of 1-alkoxyethyl groups, which have a high resolving power, also has problems. As a result of PED, the pattern profile varies to narrow the line width in the unexposed area (slimming). When the focal point is offset on a stepped substrate, the pattern on the mask cannot be accurately transferred to the resist film. Specifically, although a rectangular pattern is obtained on accurate focusing, any offsetting of the focal point results in the pattern top being noticeably reduced, failing to keep rectangularity. Then the depth of focus is of significance when it is desired to produce a finer pattern. If the depth of focus is narrow, it becomes impossible to form an accurate pattern on a stepped substrate is difficult, and hence to fabricate microelectronic devices by relying on pattern transfer through etching.
Even when a resin having acid labile groups of at least two types such as t-butoxycarbonyl and 1-alkoxyalkyl groups is used in a resist composition, no satisfactory results are obtained with respect to the above-described problems, especially resolution and focal latitude.